Orbitally forced sea-level changes in the upper Turonian–lower Coniacian of the Tethyan Himalaya, southern Tibet

Abstract

Although the mid-Cretaceous is considered to be a typical interval of greenhouse climate and high sea level, cooling events associated with regressions were inferred in recent years. We conducted a biostratigraphic, chemostratigraphic, sequence stratigraphic and cyclostratigraphic investigation of upper Turonian–lower Coniacian marine strata in the Tethyan Himalaya zone, to retrace the sea-level variations and to clarify their global correlations. According to the planktonic foraminiferal zonation, the studied interval is part of the late Turonian–early Coniacian Marginoruncana sigali and D. concavata Zones. The carbon isotope curve shows a good correlation to reference curves in the Boreal and western Tethys realms with all major and minor late Turonian δ13C events identified, indicating that the C-isotope curve provides an excellent tool for global stratigraphic correlation in the Turonian. Based on the lithological variations of clastic input and physical and chemical proxies, the succession is divided into two third order and eight fourth order sequences. Spectral analysis indicates that fourth order sea-level changes were linked to the astronomically stable 405-kyr eccentricity cycle. Comparison with classic global sea-level curves, we suggest that late Turonian–early Coniacian sea-level changes along the southeastern Tethyan margin were controlled by eustasy. The significant regressions during ∼90–89.8 Ma and ∼92–91.4 Ma, which are recorded in different continents, may be interpreted as the result of continental ice expansion, giving some support to the notion that ephemeral polar ice sheets existed even in the super-greenhouse world.